Murphy A252 No FM

[JohnRob]

Hi,I am in need of some assistance in getting the FM functioning by someone familiar with this A252 circuit. When I bought the set I assumed that I was going to replace some Hunts caps and all would be well but I have found that the 2nd FM IFT was O/C in three places on the primary winding.

I wondered what would caused this and have carefully unwound it and soldered it together (hardly very satisfactory I know) hoping there would be signs of life.

I guess it would need a new winding and then realignment.

Should I give up now due to the complexity of this repair?

I would be grateful for any comments.

John

[ukcol]

Hello John

Welcome to the forum.

Check out your radio as far as you can to make sure there are no other nasty surprises, then send a PM (private message) to murphymad (Mike Barker) and ask his advice. He may be able to rewind it for you or even supply you with a replacement from a scrap set.

[JohnRob]

Hi Colin,
Thank you for the prompt reply.
I will do as you suggest and send a PM to to Murphymad, Mike Barker.
I would love to get the set running FM but my concerns now are why the IFT would go O/C in three places and then of course I guess it will need re alighnment and I don't have the sig gen.
Thanks again and Kinf Regards
John Holley

[paulsherwin]

They usually go because of corrosion, though I agree that 3 breaks is very unusual.

It's possible to realign the IF without a wobbulator and scope using just off air signals, though it's tricky.

[JohnRob]

Hi Paul the 3 breaks did look as though they had corroded I unwound it and soldered them,which wasn't very satisfactory and of course still no joy.

Is it worth me trying to adjust the IF core with the botched up repair or leave well alone till I replace or rewind it?

I have sent a message to Murphymad Mike to see if he has any advice,
Kind Regards John Holley

[paulsherwin]

You should get something on FM even if one of the transformers is misaligned. I think you have an additional fault somewhere.

[JohnRob]

OK Paul, thanks for that.

I will investigate further with the temporarily botched up IFT in situ.

[JohnRob]

I guess I need to start replacing the Hunts and Wax capacitors in the FM section, can anyone suggest a good place to source these?

[westcliff]

Hello John,

This thread should contain the information you are looking for:

http://www.vintage-radio.net/forum/s...ad.php?t=55701

Gus.

[David G4EBT]

Originally Posted by JohnRob

I guess I need to start replacing the Hunts and Wax capacitors in the FM section, can anyone suggest a good place to source these?

Personally, I'd say leave that till later - the less you disturb at this stage the better and replacing them is most unlikely to restore FM signals.

Basically, you stand a better chance of success if you logically diagnose the fault(s), rather than to make subjective assumptions. The one capacitor that you really must replace in the audio coupling capacitor - C75, 0.01. I'd suggest that you first check the voltages around the valves and compare them with the circuit, noting any variances. (You need to check them with the bandswitch is two positions as explained in the data). Also, check the continuity of the band-switch around the FM section, and in any event, clean the contacts with De-Oxit or Servisol if you haven't already done so.

Have you listed the resistors and measured their actual values and compared them with the original value per the circuit? Anything within say 25% plus would most likely be OK, but if any have drifted high. For example, if R1 - 4.7 Anode/Grid feed resistor (4.7K) has drifted high it will lower the voltage on the V1, which should be around 150V.

Have you checked that you have continuity of the repaired IF coil? If so, it doesn't follow that it won't work just because it may not look pretty. But yes, Mike Barker is the man to go to in this respect. It's not for nothing that his forum name is 'Murphy Mad'!

As to FM alignment, fortunately the maker's data explains this in some detail. You do need an AM signal generator that covers the ranges 150 - 30MHz for AM alignment, nd 85 - 100 MHz for AM alignment, but you don't need an FM signal generator, nor a wobbulator. You do need an analogue voltmeter with a 'high' input resistance, which the data rather amusingly refers as 'at least 20,000 Ohms per Volt, typical of the era (AVO8 etc), but fairly ordinary today. You also need a simple RF output meter, which the data shows how to make - a 50uA DC meter, a 0.01 u F cap, and a germanium diode - 1N34, OA90 or whatever.

It's worth remembering that many sets which featured FM were mostly used on FM, so when the set failed on FM, it was often pensioned off, consigned to the loft and the money that would have been spent on a service engineer was spent on a transistor AM/FM set. Often - though not exclusively, the loss of FM signals was the valve in the FM section - especially with certain valves known to be short-lived such as the ECC85. A service visit plus an ECC85 would at the time probably have bought two AM/FM transistor sets, let alone one.

If a known good 6F12 valve (V1) could be had, that at least would eliminate your own valve as being the culprit.

Have you tried an external VHF aerial - even one of the cheap 300 Ohm ribbon one, to help maximise signal strength as compared to the internal aerial?

As to replacements for the waxy paper caps, the ubiquitous yellow 630V polyester replacements are widely available from UK suppliers on e-bay, as well as from the BVWS for members.

Just a few off the top thoughts which I hope might help a bit.

Good luck in your endeavours John.

[JohnRob]

Hello David,thanks for taking the time to offer much needed advice.
I was reading on this forum that all those Hunts and wax capacitors should be replaced but I much prefer to logically find the fault as you suggest.

I have checked the continuity of my botched IFT repair and also the wavechange switch.I have connected my loft UHF radio aerial but I still don't get a squeak on FM.

I bought the Murphy Manual download and poked about with my scope and DVM and the valve voltages seem roughly OK and I'm getting the 10 meg IF where I think it should be.

I was concerned that the IFT repair and subsequent mis alignment might not alow the broadcast signal to get through but Paul Shermin feels that something should get through even if not aligned,so I'm going to check round the valves a bit more diligently.

Strangely C75 appears missing on my Murphy manual but there is a C76 coupling cap 0.01mfd.
Am I wrong to assume that this is ok since the AM is functioning perfectly?
I am grateful for any advice
Regards John

[ms660]

Yes, C76 is the one, it must be replaced, if it goes electrically leaky it will put +ve volts on the grid of the output valve....not good.

A favourite one to replace for FM is C71 in the ratio detector, note its polarity, +ve connects to chassis, you can use a 4.7uF of a suitable voltage rating, 50v or 63v would be ok.

Lawrence.

[PJL]

There is an A242 chassis in the offered section going for free. This may have the same transformers.

[JohnRob]

Re my A252, I appear to have near correct voltages on the valves and the 10 meg IF seems to be sequentially running throughout the stages, although I cant really see any evidence of a broadcast signal. V2 anode V is a bit high at 170 v on FM and 32 V on AM.
I am in the process of replacing C 71 but I wouldn't have thought I need to replace C76 yet, given that AM works perfectly?
Grateful for any further advice.